1. Field of the Invention
The present invention relates to an optical element that extracts either circularly polarized light of a prescribed rotation direction or linearly polarized light of a prescribed polarization direction, and more particularly it relates to a polarization extraction optical element that reflects part of one of right and left circularly polarized light or one of two linearly polarized lights, the oscillation planes of which are mutually perpendicular, and passes the remaining light.
2. Description of the Related Art
A polarizing beam splitter or polarizing filter is known as an optical element that reflects either right or left circularly polarized light and passes the other light.
One type of polarizing filter is disclosed, for example, in Japanese Unexamined Patent Application publications S60-191203, S62-136602, and Japanese Unexamined Patent Application publication H2-186301.
All of the above make use of a cholesteric liquid crystal to reflect either right or left circularly polarized light of a specific wavelength and pass the other light.
As a method for broadening the wavelength band of the passed light, there is, for example, the disclosure in Japanese Unexamined Patent Application publication H6-281814, in which the helical pitch of a cholesteric liquid crystal is changed, or as disclosed in Japanese Unexamined Patent Application publication H9-304770, in which several layers of cholesteric liquid crystals having different helical pitches are overlapped.
An example of a linear polarization extraction optical element would be a polarizing beam splitter or a polarizing filter. These have a planar multilayer structure in which a large number of layers of birefringent materials are laminated, arranged so that the difference in refractive indices of layers that are mutually adjacent in the thickness direction with respect to one of two linearly polarized light components that have oscillation directions that are mutually perpendicular within a plane is different than the difference in refractive indices of layers mutually adjacent in the thickness direction with respect to the other linearly polarized light component.
An optical element that uses a cholesteric liquid as any of those described above reflects either right or left circularly polarized light in a specific wavelength range and passes the other circularly polarized light component in this wavelength range, and also passes all of the left and right circularly polarized light in other wavelength regions, and light incident from the opposite side is affected in the same manner, either left or right circularly polarized light being reflected and therefore striking the optical element, the result being that, although it is possible extract as reflected light either right or left circularly polarized light in a specific wavelength region, it is not possible to perform extraction from the transmitted light.
A reflecting/transmitting optical element using cholesteric liquid crystal has a fixed reflection/transmission ratio of 50/50, and is therefore accompanied by the problem of preventing free changing of this ratio as required.
Additionally, because either right or left circularly polarized light is passed and the other light is reflected, meaning that the light is separated in accordance with its polarization direction, the extracted light is unnatural light made up of a single polarization component.
For this reason, for example, in the case of using this optical element as a polarizing filter for photographic equipment such as a camera or the like, depending upon the object being photographed, there is the problem of unexpected fringe patterns occurring.
Additionally, when a linearly polarization extraction optical element is used as a polarizing beam splitter or the like, the reflection/transmission ratio is fixed at 50/50, and cannot be changed freely as required.
On one side of the element, it possible to extract either reflected light or transmitted light of the incident light made up by a component of one linear polarization only, it is not possible to extract from both the reflected light and the transmitted light two linearly polarized components having oscillation planes that are mutually perpendicular.
Because one linearly polarized component of two components having mutually perpendicular oscillation planes is transmitted and the other is reflected, that is, because the light is separated in accordance with the polarization direction, the resulting light is unnatural, being made up of a light component of only one polarization direction.
For this reason, when such a beam splitter is used as a polarizing filter in photographic equipment such as a camera, depending upon the object being photographed, there is the problem of unexpected fringe patterns occurring.
The present invention was made in consideration of the above-described drawbacks of past technology, and has as an object to provide a polarized light extraction optical element that can extract, from both reflected light and transmitted light, either one of right- and left-rotational circularly polarized light of a specific wavelength region, or one of two linearly polarized light components of a specific wavelength region, the oscillation planes of which are mutually perpendicular.
It is another object of the present invention to provide a polarized light extraction optical element that enables the ratio between a transmitted light component and a reflected light component to be arbitrarily adjusted.
It is yet another object of the present invention to provide a polarized light extraction optical element that, by mixing different polarized light components, can obtain light that approaches to being natural light.